1. Field of the Invention
Plastic materials having stable properties under conditions of ultraviolet light and high temperatures in the presence or absence of oxidizing atmospheres.
2. Description of the Prior Art
Heretofore, in the preparation of ultraviolet stable polymer films and coatings, a conventional practice has been to mix within the matrix or to coat the surface of the polymer with an ultraviolet absorbing compound, or opaque filler such as carbon black, talc, clay or a pigment.
Although this invention is not predicated on its theory of operation, it is believed that the poly (diphenylsiloxy) arylazines of the present invention derive their photostability from the contribution of the repeating units to the rapid deactivation of electronically excited states, a process which is believed to allow the polymer to relax to the ground state at a rate faster than any bond scission process.
Many of the protective compounds of the prior art are low molecular weight substances which are incorporated in the polymer melt. coated on the surface of the polymeric structure or dissolved in the casting solvent. While many of the substances involved are effective when used in relatively thick layers or high concentrations, the results obtained therewith are far from optimum because the incorporation of 0.01 to 10% by weight of stabilizer often adversely affects the physical, chemical and optical properties for which the polymeric materials were initially chosen. Moreover, conventional techniques of protection from ultraviolet have been tailored to terrestrial conditions and are often deficient in the space environment. For example, the commercially employed benzophenone derivatives actually become sensitizers for the photodegradation of polymers in vacuum at wavelengths below 3000 A.
A further deficiency of commercial ultraviolet absorbers is that as additives they show a tendency to be leached out of plastics by solvents, and to migrate to the surface or to be lost by evaporation during the hot molding operations and other thermal processing to which plastics may be subjected. This loss of absorber is accompanied by a loss of protection against photodegradation.
The use of copolymerizable groups as the ultraviolet stabilizer has also been suggested as a means of covalently binding the stabilizer in the polymer chain. However, in practice only a small fraction of the copolymerizable stabilizer is actually incorporated in the polymer chain, leaving the unreacted stabilizer to be leached out in the same manner as the additives mentioned above. Since the randomly incorporated stabilizer constitutes only a small fraction of the polymer chain, no protection is afforded to long stretches of the chain that are devoid of stabilizer groups.
Many of the commercial polymers that have a degree of photostability are not soluble in common solvents and therefore cannot be used economically in paints, coatings or films.
Heretofore, 2,2'-dihydroxynaphthalazine (2,2'-DHNZ) has been described [N. Kaistianpaller & D. DuHon, Appl. Optics 3,287 (1964)] as a yellow organic phosphor with constant fluorescent effeciency between 1000 and 3500 A, but there has not been any recommendation for the use of such compounds as a repeating mer unit in an alternating copolymer with diphenylsiloxane units to enhance the ultraviolet stability of a polymer.
Alternating copolymers composed of aryldioxy and diphenylsiloxy repeating units have been prepared by the melt condensation of aryldiols and bis-(anilino)diphenylsilane. [J. E. Curry & J. D. Byrd, J. Appl. Polymer Sci., 9, 295 (1965)] Although the resulting polymers displayed good thermal stability up to 300.degree.C and coatings cast from tetrahydrofuran had considerable ultraviolet stability in vacuum, they did not contain the diazine linkage, (--CH=N--N=CH--) as the basic repeating unit which is essential to achieve the objects of the present invention.
Sulfonated derivatives of 2,2'-dihydroxynaphthalazine have been described [U.S. Pat. No. 2,818,432] as useful fluorescent dyes for animal fibers or polyamides. However, the resulting dyed polymers did not contain a diazine linkage as the basic repeating unit of the polymer backbone because the 2,2'-dihydroxynaphthalazine group was attached to the protein or polyamide as a pendant side-chain through the sulfonic acid group.
Polymers derived from hydrazine have been described [U.S. Pat. No. 2,957,852] as elastomers that exhibit improved stability to ultraviolet light. However, the repeating units of these polymers were ##EQU1## linked through carbonyl groups where X and X' represent hydrogen or a monovalent organic radical. These elastomers, although derived from hydrazine, do not contain the diazine linkage essential to the present invention.
Heretofore, polymers based on the reaction of the hydrazine class of compounds and halosiloxanes have been described [U.S. Pat. No. 2,877,201] as being water repellent compounds useful for application to aircraft windshields. However, since this reaction is limited to linear aliphatic hydrazine compounds having at least one functional hydrogen atom attached to each nitrogen atom, it is not possible to prepare by this method polymers which have the diazine linkage as the repeating unit in the polymer backbone as described in the present invention.